Global Positioning System (GPS) receivers acquire GPS radio signals from several satellites to compute position. The process of acquiring the GPS radio signal is enhanced in speed and sensitivity if a GPS receiver has prior access to a model of the satellite orbit and clock. This model is broadcast by the GPS satellites and is known as ephemeris or ephemeris data. The ephemeris is presented to receivers as part of the GPS signal or navigation message. The broadcast ephemeris comprises navigational information for the transmitting GPS satellite. The navigational information may comprise standard satellite orbits models, clock model, and/or information about the operation status of the GPS satellite (healthy or unhealthy), which may be essential for determining position and velocity of a GPS receiver.
The broadcast ephemeris is encoded within the navigation message and is transmitted at a rate of 50 bps, taking 18 seconds in all for a complete ephemeris transmission. The broadcast ephemeris is typically valid for 2 to 4 hours into the future (from the time of broadcast). Before the end of the period of validity, the GPS receiver needs to obtain a fresh broadcast ephemeris to continue operating to produce an accurate position. It is slow (no faster than 18 seconds), frequently difficult, and sometimes impossible (in environments with very low signal strengths), for a GPS receiver to download ephemeris from a satellite. For these reasons, AGPS (Assisted-GPS) technique may be used to provide ephemeris assistance data from an AGPS server to speed up the process of determining a position fix for the GPS receiver, especially in a weak signal environment. The ephemeris assistance data may be derived at the AGPS server from satellite signals collected via a plurality of reference GPS receivers. The ephemeris assistance data remains valid for only a few hours. However, the AGPS sever may be enabled to provide the GPS receiver with ephemeris assistance data valid for up to, for example, 10 days, by using Long Term Orbits (LTO) technology. The utilization of the LTO technology enables the benefits of AGPS technology even when temporarily out of mobile operator network range.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.